Aluminum trihalide gases, for example aluminum trichloride (AlCl3) gas and aluminum tribromide (AlBr3) gas, are used as raw materials for manufacturing a group III nitride single crystal including aluminum (aluminum-based group III nitride single crystal). The aluminum-based group III nitride single crystal, whose band gap energy is large, enables emission of short wavelength light in the ultraviolet range. Therefore the crystal is expected to be used for ultraviolet light emitting diodes for white light sources or for sterilization, laser for read/write of high density optical disc memories, laser for communication, and the like. Thus an aluminum trihalide gas of high purity which can be used for manufacturing an aluminum-based group III nitride single crystal is required.
As a method for manufacturing an aluminum-based group III nitride single crystal using an aluminum trihalide gas, Hydride Vapor Phase Epitaxy (HVPE) is known. HVPE is a method of manufacturing a crystal by conducting a reaction of, for example, an AlCl3 gas and a nitrogen source gas (e.g. ammonia gas) on a base substrate. This method is suitable as a method of growing a thick film of a single crystal.
An aluminum trihalide gas used for this HVPE method is manufactured as follows. Specifically, the gas is manufactured by a method of conducting a reaction of aluminum and a hydrogen halide gas at a temperature of 700° C. or less (see Patent Literature 1). This method makes it possible to inhibit generation of aluminum monohalide (e.g. aluminum monochloride (AlCl), aluminum monobromide (AlBr)) gases which are highly reactive with silica glass, which results in less deterioration of silica glass which is a material of HVPE devices. Patent Literature 1 describes manufacturing an aluminum-based group III nitride single crystal by flowing an aluminum trihalide gas obtained by the above method, into a crystal growth zone (on a base substrate) heated to a temperature of no less than 700° C. and no more than 1300° C., and reacting the gas with a nitrogen source gas.
According to the method described in Patent Literature 1, it is possible to inhibit generation of aluminum monohalides. However, as described in Patent Literature 1, an aluminum monohalide gas corresponding to the equilibrium state is inevitably generated, even in a tiny amount. The higher the generation temperature is, the relatively more the supply of the aluminum monohalide is. If the supply of the aluminum monohalide gas increases, the crystal quality of the aluminum-based group III nitride single crystal may degrades.
In a case where an aluminum monohalide exists as a component of an aluminum halide which is a raw material, it is known that deposition of a solid proceeds with a temperature decrease in the transportation of the aluminum monohalide gas as a driving force, which results in the deposition of metal aluminum (see Patent Literature 2 for example). In a case where the solid deposition occurs a lot in the transportation of a raw material, deposits accumulate inside pipes over time, and may peel off at an unintended timing. Further, if the peeled deposits attach to the growth surface of the aluminum-based group III nitride crystal, an abnormal growth starts from the attached deposits, which may result in degradation of the crystal quality of the aluminum-based group III nitride crystal.
In the method of Patent Literature 2, the solid deposition from the aluminum monohalide gas is intentionally promoted by: bringing aluminum and a halogen-based gas into contact with each other to form a mixed aluminum halide gas including an aluminum trihalide gas and an aluminum monohalide gas; thereafter making the mixed gas pass through an area (conversion part) having temperature lower than the temperature at which the contact was made. This method decreases the amount of the aluminum monohalide existing in the gas and inhibits the solid deposition inside pipes during the transportation of the raw material after the gas passes the conversion part.
However, with the method described in Patent Literature 2, a precise temperature control is required in the generation process and the transportation process of the aluminum halide gas. In addition, in order to more certainly remove the aluminum monohalide which causes the solid deposition, it is necessary to arrange fillers and the like, which makes the device complicated.